An axle is generally described as a supporting shaft for a rotating wheel(s) or gear(s). Axles are used in many different environments, including in automobiles and aircraft. In general use, an axle may be required to sustain varying weights placed upon it and therefore its structural integrity may be important to its lifespan. For example, in an aircraft application there will be an increase in the load placed on the axle when the plane is stationary and being loaded with passengers, cargo and fuel. An even greater load will be placed on the axle when the wheels and the axle to which they are attached come into contact with the runway upon landing. It is therefore desirable to monitor the condition of the axle to ensure that it is not damaged or in need of servicing or maintenance.
Knowing the forces applied to aircraft landing gear axles provides for the determination of aircraft weight and balance, which is of interest to aircraft operators. The weight (mass of the aircraft, fuel, occupants, and cargo) and balance (the position of the centre of gravity of the aircraft) are critical factors that require measurement or calculation prior to every flight. Currently, almost every aircraft departs using calculated weight and balance values. These calculated values are based on average weights, not the actual weights of passengers and baggage, so aircraft operators must limit the usage of their aircraft to a narrower band of weight and balance values than that set by the aircraft manufacturer. This limits the utilisation of the aircraft, and reduces its potential revenue. In addition, the calculations are performed manually in some instances, and in others by central calculation departments. If a method of measuring the weight and centre of gravity existed that could reliably determine these values, the usage of the aircraft would increase (more passengers/cargo) could be carried, and the costs to aircraft operators to determine the values could be significantly reduced.
A number of attempts to determine the weight of aircraft have been tried, with various degrees of success. The benchmark are stationary scales that an aircraft rolls onto, such that each landing gear or landing gear wheel is weighed. This method provides the standard to which all others are compared, but since the scales are not carried aboard the aircraft, and since the weighing procedure typically takes a significant amount of time, this method is not appropriate for the determination of the aircraft weight and balance prior to each flight. A number of flyable approaches have been attempted. In all these methods the landing gear, or portions thereof, form the element on which the measurement will be made since the landing gear, and its associated wheels and tires, are where the aircraft's weight is reacted by the ground. One of the earliest approaches to determining the weight over a landing gear was by measuring the pressure of the gas in the gas spring that supports the aircraft. This method suffers from a lack of accuracy due to the friction of the gas and oil seals in the strut which carry some of the load. Methods exist (Nance) to account for this friction, but these are either based on empirical data or require complicating the gas oil strut of the landing gear with various valves, tubes, and actuators that by their existence reduce the reliability of the landing gear system.
Other methods have been tried that more directly relate to the present invention—they work by attempting to measure the deflection of the landing gear axles. A direct approach uses strain gauges, either bonded to the axle, or bonded to a sensor fitted within the axle. Strain gauges use conductive metal that when stretched or compressed will cause an increase or decrease in electrical resistance across the material. The amount of change in the electrical resistance can be used as a measurement of the strain or deflection that the component to which the strain gauge is attached to is under. Such gauges have limitations based on the constriction of the elastic limits of the material used and the lack of high accuracy that can occur in the measurement readings. In addition, strain gauged based systems suffer from a lack of longevity in landing gear applications related to their reliance on mechanical bonding (gluing) of the gauge to the area of interest. Another point of failure of strain gauges is through corrosion where the electrical leads are terminated to the gauge. These terminations are by necessity in a harsh environment (the aircraft landing gear axle) and typically do not survive long in service.
A further attempt to measure the shear deflection of the axle has been fielded. This system employs a variable reluctance sensor which operates by measuring directly the displacement of the landing gear axle. The sensor is bolted to specially machined lugs on the exterior of the landing gear axle. In practice the system is expensive due to the requirement for machining lugs on a part which would be normally have been turned in a lathe and difficult to calibrate and use.
Other systems, which have been contemplated or demonstrated, include systems that directly measure material properties of a component to which they are attached, i.e. the axle. Such measurements (such as Barkhausen noise and other magnetic domain measurements) are then compared to predetermined material measurements and can be used to determine any potential stress on the component material. Many of these systems are experimental and have not had their reliability proven. In addition, there are questions as to how certain material properties of interest to these sensors change naturally with time. For instance, the Barkhausen noise properties of steel may change naturally over the life of a landing gear, confounding the original calibration.
In addition to the interest in measuring the weight of an aircraft, it is of interest to measure the forces acting on a landing gear in order to better determine the structural life and integrity of said landing gear. A method to measure axle deflections could provide a significant amount of information towards the determination of landing gear structural life.